The present invention relates to radiotherapy systems using ions for the treatment of cancer and the like and, in particular, to a phantom for such systems.
External beam radiation therapy may treat a tumor within the patient by directing high-energy radiation in one or more beams toward the tumor. Recent advance external beam radiation systems, for example, as manufactured by Tomotherapy, Inc., treat the tumor with multiple x-ray fan beams directed at the patient over an angular range of 360°. Each of the beams is comprised of individually modulated rays whose intensities can be controlled so that the combined affect of the rays over the range of angles provides an arbitrarily complex treatment area with minimized skin dose.
The benefit of the improved accuracy possible with such systems is ensured by careful characterization and monitoring of the x-ray beam geometry and intensity, for example, through portal imaging devices and entrance dose monitors.
X-rays expose tissue not only within the tumor but also along the path of each ray into and out of the patient. While judicious selection of the angles and intensities of the rays of x-ray radiation can limit radiation dose outside of the tumor, a desire to more closely conform the radiation dose to the tumor has raised interest in substituting ions such as protons for x-ray radiation. Unlike x-rays, the dose deposited by a proton beam is not uniform in homogenous tissue, but rises substantially, at the “Bragg peak” just before the proton stops within the tissue. Further, because the proton can be controlled to stop within the tissue, exit does from the proton beam can be substantially eliminated. These two features allow improved placement of dose within the tumor.
Unfortunately, unlike x-rays, ions are not easily characterized by entrance dose monitors and portal imaging device. In part, this is because such devices are sensitive largely to flux (photons per unit time) as opposed to radiation energy, and it is this latter characteristic which determines the important quality of proton range. Portal imaging systems, which rely on radiation exiting the patient, are of limited value in a proton imaging system in which the protons stop within the patient.